byiafu is an abbreviation of 'By you, I am forever undone.'
When Cardan Greenbriar said this line to Jude Duarte, he was expressing himself by saying that Jude's influence on him is so profound on him like no other that he's forever changed by their connection. Cardan acknowledges his feelings and the impact Jude has had on his life. This statement also represents his acceptance of his feelings towards Jude. Cardan has previously shielded himself from emotional attachment to protect against potential hurt or manipulation. However, by acknowledging that he is "forever undone" by her, Cardan admits that he has opened a gate and allowed himself to be emotionally vulnerable and open to love. Their relationship throughout the book, The Wicked King, was different from both perspectives. Jude had always thought that Cardan is trying to trick her, deceive her, exploit her, because that's the belief system she was raised with and had also witnessed when it comes to fae. However, Cardan wanted to show Jude that she could trust him, he wanted to show her that he could trust her he wanted to work alongside her, to be her equal.
Furthermore, the phrase "forever undone" implies that Jude has unraveled the carefully constructed version of Cardan which he presented to the world, she has broken down his emotional barriers.
Overall, "By you, I am forever undone" is a heartfelt declaration from Cardan to Jude.
byiafu = "By you, I am forever undone." Which is said by Cardan Greenbriar, from The Folk of Air trilogy by Holly Black, to Jude Duarte.